Determination of tropospheric vertical columns of NO2 and aerosol optical properties in a rural setting using MAX-DOAS
1Centre for Atmospheric Chemistry, York University, Toronto, ON, Canada
2Satellite Group, Max Planck Institute for Chemistry, Mainz, Germany
3Air Quality Research Division, Environment Canada, Toronto, ON, Canada
4Ontario Ministry of the Environment, Toronto, ON, Canada
5School of Energy and Environment, City U, Hong Kong, China
*now at: Department of Chemistry, University of Toronto, Toronto, ON, Canada
Abstract. Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed in a rural location of southwestern Ontario during the Border Air Quality and Meteorology Study. Slant column densities (SCDs) of NO2 and O4 were determined using the standard DOAS technique. Using a radiative transfer model and the O4 SCDs, aerosol optical depths were determined for clear sky conditions and compared to OMI, MODIS, AERONET, and local PM2.5 measurements. This aerosol information was input to a radiative transfer model to calculate NO2 air mass factors, which were fit to the measured NO2 SCDs to determine tropospheric vertical column densities (VCDs) of NO2. The method of determining NO2 VCDs in this way was validated by comparison to composite VCDs derived from aircraft and ground-based measurements of NO2. The new VCDs were compared to VCDs of NO2 determined via the satellite instruments SCIAMACHY and OMI, for overlapping time periods. The satellite-derived VCDs were higher by 50%, with a mean positive error of 0.5–0.9 × 1015 molec cm−2. This last finding is different from previous studies whereby MAX-DOAS geometric VCDs were higher than satellite determinations, albeit for urban areas with higher VCDs. An effective boundary layer height, BLeff, is defined as the ratio of the tropospheric VCD and the ground level concentration of NO2. Variations of BLeff can be linked to time of day, source region, stability of the atmosphere, and the presence or absence of elevated NOx sources. In particular, a case study is shown where a high VCD and BLeff were observed when an elevated industrial plume of NOx and SO2 was fumigated to the surface as a lake breeze front impacted the measurement site. High BLeff values (~1.9 km) were observed during a regional smog event when high winds from the SW and high convection promoted mixing throughout the boundary layer. During this event, the regional line flux of NO2 through the region was estimated to be greater than 112 kg NO2 km−1 h−1.